1. Field of the Invention
The present invention relates to a base station combining a plurality of channel information divided as a unit of transmission power control from all the transmission channels and transmitting downlink data to mobile stations, and a transmission power control method, in mobile communication systems.
2. Description of the Related Art
Heretofore, in mobile communication systems based on Code Division Multiple Access (CDMA), transmission power control procedure which controls a transmission power to an optimal value based on reception quality is performed in downlink and uplink radio communication channels between a base station and mobile stations. Transmission power control of downlink is achieved in the mobile station by monitoring reception quality and sending control information on the basis of the reception quality to the base station. Also, transmission power control of uplink is achieved in the base station by monitoring reception quality and sending control information on the basis of the reception quality to the mobile station.
Particularly, as to the transmission power control of downlink, the base station increases transmission power to the mobile stations in order to maintain required communication quality in a situation that a number of call channels increases, a radio communication environment deteriorates, or the mobile station gets away from the base station. Increment of the transmission power causes waveform distortion in a transmission power amplifier. Therefore, the base station needs to limit the transmission power so that a total transmission power value to transmit downlink data to the mobile stations does not exceed a maximum transmission power value (upper limit of the total transmission power permitted) set up previously.
A technology of limiting the transmission power in the base station is disclosed in EP 1199819 A1. The base station controls a variable attenuator connected to input side of a transmission power amplifier in order to protect the transmission power amplifier, when the transmission power has approached an output power limit of the transmission power amplifier.
Next, a description will be given in more detail of the base station. FIG. 6 is a block diagram showing a configuration of a conventional base station.
In FIG. 6, the conventional base station 101 includes a channel combining unit 102, a digital-to-analog converter (D/A) 103, a mixer (MIX) 104, a variable attenuator 105, an amplifier (AMP) 106, a divider (HYD) 107, a detector 108, an analog-to-digital converter (A/D) 109, a controller 110, a transmission power amplifier (TPA) 111, and a local oscillator 112.
The channel combining unit 102 carries out additive combine of each of received signals (a pilot channel, a control channel, and a plurality of dedicated channels), calculates total power value from the combined signal, and outputs as total power information to the controller 110.
The controller 110 compares the total power information with maximum transmission power information set up previously. Next, the controller 110 compares the maximum transmission power information with the transmission power information (output power of the amplifier 106) output from the analog-to-digital converter 109, when the total power information has exceeded the maximum transmission power information. Furthermore, the controller 110 controls attenuation of the variable attenuator 105 connected to the input side of the amplifier 106 based on the comparative result. That is, the base station can protect the transmission power amplifier 111 by maintaining the transmission power information within the maximum transmission power information of upper limit.
However, in the base station disclosed in EP 1199819 A1, even if there is a transmission channel whose power fluctuation should be avoided, there is a problem that the whole transmission power is attenuated uniformly.
Aside from this conventional technology, in advance of this application, the applicant has proposed the base station which maintains a transmission power of specific transmission channel and can limit the total transmission power (see, U.S. patent application Ser. No. 11/137,384 (US2005/0265292 A1) and European Patent Appl. No. 05090159.4 (EP1603251 A1) ).
The base station classifies all the transmission channels into a plurality of power classes (for example, a common control channel to all the mobile stations, a communication channel allocated to each of mobile stations, and so on) as a unit of transmission power control, and performs transmission power control for controllable power class. That is, the base station controls the transmission power of the controllable power class in which reduction of the transmission power is allowed, without reducing the transmission power of the control channel to avoid fluctuation of transmission power.
However, when communication channel allocated to each of mobile stations is scheduled to controllable power class of transmission power, there is a problem that the entire transmission power is attenuated uniformly, regardless of communication quality with which each of the mobile station is provided.
As described above, the conventional technologies may affect the quality of the entire communication channels allocated to each of the mobile stations, when the transmission power of the base station has exceeded the maximum transmission power information of upper limit set up previously.